Fiber optical transmission filter with double-refraction element

ABSTRACT

In manufacturing optical transmission filters having double-refraction elements, extreme care is usually necessary to provide the desired phase delay. To simplify the manufacturing process while still achieving accurate phase delay, a filter is provided with at least one double-refraction element comprising a single-mode optical filter mounted between polarizers. The double refraction of the optical fiber is sufficiently weak so that the λ length within which light beams propagating with orthogonal polarization states in the fiber are mutually delayed by 2π, is at least 1 cm. In one embodiment, the optical fiber comprises alternating sections which produce linear double refraction with sections which produce elliptical double refraction.

FIELD OF THE INVENTION

The invention relates generally to an optical transmission filter and,more particularly, to an optical transmission filter containing adouble-refraction element.

BACKGROUND OF THE INVENTION

Double-refraction optical transmission filters can consist of adouble-refraction crystal plate arranged between two parallel linearpolarizers. In such an arrangement the filter possesses thecharacteristic feature that the first polarizer in the direction ofpropagation of the light determines the polarization state with whichthe light of a wide-spectrum beam impinges upon the crystal. Because ofthe double refraction, which can be assumed generally without limitationto be a linear double refraction, the light beam is split into two beamspolarized orthogonally and propagating at different velocities in thecrystal. The light beam which issues from the crystal, and which resultsfrom the superposition of the two partial beams, exhibits generallyelliptical polarization. The second polarizer, on the output side, istransparent only to the light beam component which is parallel to itsplane of polarization and its transmission varies sinusoidally with thephase delay applied to one partial beam relative to the other because ofthe crystal double refraction. Since--without consideration of thevariation in the effective index of refraction determined by the crystalmaterial dispersion--said delay is approximately in inverse proportionto the wavelength of the incident light, the transmission issinusoidally dependent on the inverse wavelength.

For a given wavelength λ of the light injected into thedouble-refraction crystal with a defined linear polarization state, thetransmission is maximum in such a filter when the relative phase shiftis 2π or an integral multiple ρ of 2π, determined by the difference inthe two optical distances travelled by the two orthogonal polarizationstates. This can be written as the equation: ##EQU1## where p is theorder in which the filter is operated at given wavelength λ.

This condition is met also for proximate wavelengths λ₁ =λ+Δλ₁ and λ₂=λ-Δλ₂ when: ##EQU2## The wavelength intervals Δλ₁,2 for thetransmission maxima of the filter transmission maximum for wavelength λis then given by the equation: ##EQU3##

It is apparent that a given minimum distance Δλ between the transmissionmaxima of such a filter can be maintained only when the order in whichthe filter is operated is not larger than a maximum p_(max) obtainedfrom equation (3) for wavelength interval Δλ₁ of the "longer" wavelengthwhen this equation is solved according to p: ##EQU4## This means thatsuch a transmission filter, which for a wavelength λ of 480 nm is tohave an interval Δλ of about 20 nm between transmission maxima, must beoperated with a maximum order of p=25.

From equation (1) it follows directly that a calcite plate (Δn=about0.16) with plane-parallel surfaces mounted as a double-refractionelement in a transmission filter operated at a wavelength λ=500 nm withorder 25, must present a thickness d of about 0.081 mm. For asatisfactory filter operation accuracy of about 1/100 of λ, thicknessd.sub.λ within which a phase delay 2π occurs between the two orthogonalpolarization states, is required, i.e. 30 nm or 1/17 of the wavelength.Therefore, the crystal plate must be processed with high accuracy, whichnaturally entails very high manufacturing costs for the filter. This istrue also when, instead of a calcite plate, a quartz plate is used as adouble-refraction element in which the difference Δn between the indicesof refraction applicable to the two orthogonal polarization states isabout 0.01. In this case the λ thickness is about 50,000 nm, and thetolerance acceptable for the quartz-plate thickness is about 500 nm,which is thus on the order of magnitude of the wavelength of the lightto be filtered.

This disadvantage in the difficult and expensive production of suchcrystal plates applies particularly to filter arrangements in which aplurality of crystal plates are mounted in succession as a stack alongthe beam path. The first multilayer arrangement of this type was afilter proposed by Lyot (B. Lyot, Ann. Astrophys. 1944:7(1), 2). TheLyot filter comprises, for example, N plates stacked successively in thedirection of light propagation, each plate being used with double thethickness of the preceding plates. Each plate is mounted with thepolarizers crossed at a right angle. The optical axes of thedouble-refraction delay plates extend at 45° to the planes ofpolarization defined by the polarizers. The resulting transmissionpresents very definite transmission maxima with a stop-band which isdetermined by the plates of least thickness, and whose bandwidthdecreases as the number of plates increases. Weakly marked secondarymaxima also exist between the primary maxima. The transmission bandwidthobtainable with a Lyot filter comprising up to 10 plates is typically 5to 0.5 A.

Similar narrow bandwidths are obtained with the multilayerdouble-refraction transmission filter suggested by Solc (I. Solc,Czechoslov. Cosopis pro Fysiku 1953:3, 336; 1954:4, 607, 609; 1955:5,114). In a structure roughly equivalent to that of a Lyot filter with Nplates, the Solc filter comprises, for example, m plates of equalthickness d equal to the thickness of the thinnest plates of the Lyotfilter. The entire stack of plates is arranged between only twopolarizers. The optical axes of the individual crystal plates areparallel to the plate surfaces and perpendicular to the direction oflight propagation. In a first embodiment of the Solc filter the opticalaxes of the individual crystal plates are shifted fanlike by an angle

    ω.sub.j =(ξ/2)+(j-1)ξ                          (5)

with

    ξ=π/2m                                               (6)

where m is the number of plates of equal thickness. The polarizers areparallel.

In a second equivalent embodiment the directions of the optical axesalternate successively at an angle

    ω.sub.j =(-1).sup.j+1 (ξ/2)                       (7)

where equation (6) applies to ξ. The polarizers between which the stackof plates is arranged are crossed.

A further description of other properties of the Lyot filter and theSolc fan and folded filters can be found in a comparative description byJohn W. Evans (Journal of the Optical Society of America, Vol. 48, No.3, March 1958, pp. 142ff.).

It is apparent that, because of the multiplicity of necessary crystalplates which must be processed with the above-cited accuracy, and theneed to maintain exactly the orientation of the crystal-plate opticalaxes, the production of both the Lyot filter and the Solc filter isextremely complicated and expensive. Therefore, it is very difficult totune such filters.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide filters ofthe above-cited type, but which are considerably simpler and cheaper tomanufacture, and which users experienced with optical apparatus canadjust to the appropriate transmission range.

To achieve this and other objects, the present invention provides anoptical transmission filter having at least one double-refractionelement mounted between polarizers determining characteristicpolarization states. The dimension of the double-refraction element,viewed in the direction of the light path, is sufficient to obtain aphase delay of at least 2π between orthogonal polarization states for atleast one wavelength of the injected light. In particular, thedouble-refraction element is a single-mode optical fiber whose doublerefraction is sufficiently weak so that the length within which thelight beams propagating with orthogonal polarization states in the fiberare mutually delayed by 2π is at least 1 cm.

In comparison to known filters comprising one or more double-refractioncrystal plates the present invention provides for the advantage that theuse of weakly double refractive optical fibers in which the minimumvalue of the λ length-comparable thickness of the crystal material usedin known double-refraction filters is about 1 cm presents no specialdifficulty in observing a range advantageous to the processing of suchfibers to transmission filters, and especially the tolerance of about1/100 of the λ length required for the fiber length. There is also nodifficulty in obtaining exactly the necessary fiber length of 1/10 mm bycutting the fiber to the required length or breaking off the fiber atdesignated break points by scratching the fiber surface to obtain in thefiber core a break surface extending with a very good approximation at aright angle to the fiber longitudinal axis. In a single-mode fiber core,diameters relatively small in comparison to the outer diameter areespecially advantageous, so that even when the break surfaces are notexactly plane parallel, the resulting surface curvatures in the fibercore are negligible, which is necessary to avoid distorting the shape ofthe light beam. An optical fiber of the required length needs nosubsequent processing before use in a double-refraction filter.

According to the present invention, the manufacture of narrow-band,fiber-optics transmission fibers similar to multilayer crystal filtersis considerably simplified, so that they can be mass produced at lowcost. This is significant in respect to the use of such filters isfiber-optics multiplex information transmission systems requiring aplurality of such filters.

The filter of the invention can also provide for manufacture-conditioneddouble refraction, inherent linear double refraction induced by pinchingor bending the fiber, circular double refraction caused by fibertorsion, and elliptical double refraction resulting from the combinationof linear and circular double refraction.

The present invention can also provide for fiber-optics transmissionfilters similar to the Solc fan filter by the combined utilization ofinherent linear double refraction in a single-piece fiber extendingbetween parallel polarizers, and elliptical double refraction obtainedby twisting a portion of the fiber.

Further, the present invention can apply to fiber-optics transmissionfilters similar to the Solc fan and folded filters, with the necessarylinear double refraction for such filters being obtained by pinching thefiber in suitable directions. Such filters in which the doublerefraction results from pinching the optical fiber offer the advantageof continuous adjustment by varying the pinching force in effect perlength unit.

The present invention can also apply to produce filters similar to Solcfan and folded crystal filters. In this case, the double refractionrequired for filtering is obtained by bending the single-mode fiber inthe form of loops placed so that the overall size of the filter isadvantageously small and determined substantially by the diameter of theloops.

The fiber-optics transmission filter of the present invention, or aplurality of such filters, tuned to discrete wavelengths, is ideallyapplicable to fiber-optics transmission systems to separate the lightsignals transmitted from several semiconductor lasers and injected intoa single-mode optical transmission fiber, thereby fully utilizing thehighest possible information-transmission capacity in an opticalcommunications transmission system.

BRIEF DESCRIPTION OF THE INVENTION

Other details and features of the present invention appear in thefollowing description of embodiments in reference to the drawings,wherein:

FIG. 1 shows a Solc filter consisting of m double-refraction crystalplates;

FIG. 2 shows the angular distribution of the optical axes of the crystalplates in a Solc fan filter;

FIG. 3 shows the transmission curves illustrating a Solc filter;

FIG. 4 represents a filter consisting of sectionally twisted opticalfibers according to the present invention;

FIG. 5 represents a Poincare sphere illustrating the operation of thefilter;

FIG. 6 is a view of the Poincare sphere in the direction of arrow 89 inFIG. 5;

FIG. 7 shows a solid-core optical fiber pinched between two parallelcompression jaws to produce linear double refraction in accordance withthe present invention;

FIG. 8 shows an optical fiber bent to produce linear double refractionin accordance with the present invention;

FIG. 9 shows a pinching device used to produce specific, oriented lineardouble refraction in the successive sections of an optical fiber inaccordance with the present invention;

FIG. 10 is a view of the pinching device in the direction of thelongitudinal axis of the optical fiber in accordance with the presentinvention;

FIG. 11 shows a looped optical fiber used to obtain a Solc folded fiberin accordance with the present invention;

FIG. 12 shows diagrammatically a fiber-optics communicationstransmission arrangement comprising a plurality of transmission filtersof the present invention; and

FIG. 13 shows the transmission curves of the filters of the arrangementrepresented in FIG. 12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings, FIG. 1 schematically represents a Solcfilter 20 consisting of m double-refraction crystal plates 21 and twolinear polarizers 22 and 23 which will be discussed for providingfurther background of the present invention. About 1/100 of theabove-defined thickness λ-thickness d.sub.λ of the crystal plates 21,characteristic of the light transmitted through the filter 20 at adetermined wavelength, is the required tolerance. In the arrangementshown in FIG. 1 the crystal plates 21 are set up in a stack extending inthe longitudinal direction of the central axis 24 of the light beampassing through the filter 20 between input polarizer 22 and outputpolarizer 23, with the facing plane parallel faces 27 in mutual contactand at a right angle to said axis 24. The optical axes (indicated byarrows 26 in FIG. 2) of the double-refraction crystals forming plates 21are parallel to contacting plate faces 27.

In the embodiment designated as a Solc fan filter, the two polarizers 22and 23 denote the horizonal direction of polarization marked in FIG. 1by double arrows 28 and 29, and the optical axes 26 along which there isno double refraction in crystal plates 21 are arranged fanwise aroundcentral axis 24 in the arrangement of FIG. 2, according to the aboveequation (5). The angle formed by optical axes 26 with the direction 28of polarization determined by input polarizer 22, seen in the directionof light path 24, increases by the same amount ξ=π/2 m from plate toplate. For white, unpolarized light entering the plate stack throughinput polarizer 22, the filter 20 provides for transmission which isdependent on the wavelength and whose qualitative variation isillustrated by the lower transmission curve 31 in FIG. 3. The relativetransmission T, normalized to 1, is plotted against the wavelength λ.

As shown in FIG. 3, transmission T for the filter 20 of FIG. 1 exhibitsstrongly marked, narrow-band primary maxima 32 with transmission T=1 atthe wavelength λ_(k) for which equation (1) is fulfilled with the pintegral, and whose interval on the wavelength scale is given byequations (3) and (2). Between the maxima 32, considerably less markedsecondary maxima 33 and 34 appear at approximately equidistant intervalson the wavelength scale and are separated from each other and fromprimary maxima 32 by intermediate passages through zero 36 oftransmission T.

The relative transmission of the filter 20 of FIG. 1 is representedgenerally by the following equation: ##EQU5## where the values χ aregiven by the expression ##EQU6## and ξ by equation (6), and ##EQU7##designates the half delay of the double-refraction crystal plates 21, dand Δn being as defined above.

FIG. 3 also shows a transmission curve 37 of substantially sinusoidalform which would be produced by a single crystal plate 21 instead of mcrystal plates 21 between polarizers 22 and 23. It is apparent that inthis case the transmission maxima 38 of said transmission curve 37 areseparated by the same wavelength intervals as the primary maxima 32 oftransmission curve 31 associated with multilayer Solc filters 20, butthe bandwidth is much broader and corresponds approximately to thewavelength interval between transmission maxima 38, measured between thepassages through zero 39 of transmission curve 37.

FIG. 4 represents a first embodiment of a fiber-optics transmissionfilter 40 of the present invention, with properties which are similar tothose of the known Solc filter 20 of FIG. 1. The above equations arealso applicable.

The central component of the fiber-optic transmission filter 40 is aquartz-glass, solid-core fiber 41 producing a linear double refractionof for example 100 rad/m. Optical fiber 41 is a single-mode fiber inwhich light can propagate only in two mutually orthogonal polarizationstates, which is the case when 2πd/λ(n_(k) ² -n_(M) ²)^(1/2) <2.4, wheren_(k) is the fiber core index of refraction and n_(M) the fiber surfaceindex of refraction. Preferably optical fiber 41 is a weakly guiding,single-mode fiber in which the difference between indices of refractionn_(k) and n_(M) is only about 0.5%. The fiber core diameter is about 5μm and the outer diameter about 100 μm. Optical fiber 41 follows astraight line when extended, and is fixed against torsion at a number ofclamping points 42, 43, and 44. The holding means necessary for thispurpose are symbolized in FIG. 4 by retaining rings 47 attachable atdefinite intervals to an optical bench. But, of course, in practicalcases the structure of these retaining means may be in other suitableforms to fix optical fiber 41 at intervals which will be specifiedbelow.

Clamping points 42, 43, and 44 divide optical fiber 41 into m delaysections 48, 49, and 50 and (m-1) coupling sections 51 between saiddelay sections 48, 49, and 50. The function of delay sections 48, 49,and 50 is similar to that of crystal plates 20 in Solc filter 20, andthe function of coupling sections 51 is to connect the delay sections inthe same distribution as shown in FIG. 2 in directions comparable tothose of optical axes 26 of crystal plates 21, without the need ofcutting the fibers into separate pieces.

Delay sections 49, arranged between pairs of coupling sections 51, areof equal length, which, for the given wavelength λ_(k) corresponding toa transmission maximum of filter 40, is equal to (p+1) times the λlength l.sub.λk comparable to the λ thickness d.sub.λ of crystal plates21, which is supplied by the equation:

    l.sub.λk =(λ.sub.k /Δn)                (11)

where Δn is the difference between the indices of refraction in effectfor the two orthogonal polarization states possible in the filter, andon the order of magnitude of 10⁻⁵.

The length of first delay section 48 into which white light emitted by alight source 53, received by a focusing optical system 52, and injectedwith a linear polarization state determines by the position of a linearpolarizer 54, is then equal to (p+1/4) times the λ length l.sub.λk. Thelength of the last delay section 50 must be (p+3/4) times the λ length.The coupling sections 51 are of equal length. A suitable length for thecoupling sections 51 will be discussed below. A tolerance of about(l.sub.λk/ 100) is acceptable for fiber sections 48-50.

Coupling sections 51 are imparted with elliptical double refraction bytwisting the fiber midway between clamping points 42 by a determinedangle, symmetrically to a transverse axial plane 57 extending at a rightangle to the longitudinal axis 56 of optical fibers 51. Said ellipticaldouble refraction results from the fiber inherent linear doublerefraction β and a circular double refraction α determined by torsion.Viewed in the direction of the light beam, the first portions 58 ofcoupling sections 51 exhibit a left elliptical double refraction, whilethe next portions 59 thereof exhibit a right elliptical doublerefraction. The effect of this complementary elliptical doublerefraction in coupling sections 51 and the (l.sub.λk)/(4) "extra length"of first delay section 48, the total 2l.sub.λk of the extra lengths ofdelay section 49, and the 3/4l.sub.λk of the polarization state of thelight of wavelength λ_(k) propagating in optical fiber 41, is explainedbelow in reference to FIGS. 5 and 6.

FIG. 5 represents a Poincare sphere 61 showing the possible linearpolarization states by points located on the equator 62, the twopossible circular polarization states (left circular polarization L andright circular polarization R) by the north pole 63 and the south pole64, and the also possible left or right elliptical polarization statesby other points on the surface of the sphere, above or below equator 62.

One linear double refraction present in optical fiber 41 is representedby a vector 67 located in the equatorial plane 66 and whose direction isindicated by the linear initial polarization state represented by points68 and 69, whose introduction into optical fiber 41 along its lengthwould not result in a change in the polarization state of the injectedlight. The direction 2φ_(B) originates from the axes 71 associated asinherent polarization states represented by points 72 and 73 with thehorizontal or vertical polarization of the incident light. If opticalfiber 41 is oriented so that its optical axes are 72 and 73 and if lightof a different linear polarization state represented, for example, bypoint 74, is injected into optical fiber 41, the polarization statesoccurring successively along fiber 41 progress possibly several timesalong a circle 76 extending around inherent polarization state 72, at anangular velocity β whose magnitude depends on the value of the lineardouble refraction represented by a vector 77 oriented toward point 72.

In FIG. 5 it is assumed that the progress along circle 76 occurscounterclockwise. A circular double refraction α impressed on an opticalfiber 41, for example by twisting about its longitudinal axis, or whichmay be an inherent double refraction, is represented in the drawing ofPoincare sphere 61 as a vector 78 or 79 oriented in the direction ofpolar axis 63 or 64. Vector 78, oriented toward north pole 63, indicatesa left circular double refraction, and vector 79, oriented toward southpole 64, a right circular double refraction whose value is indicated ineach case by the length of the vector. When a circular double refractionα is obtained by twisting the fiber with a determined degree of torsionπ measured in rad/m, the value of the length of vectors 78 and 79 isgiven by the equation: ##EQU8## where

    α.sub.τ =g·τ                        (13)

in which g is a constant factor ranging between 0.13 and 0.16 for normalquartz fibers.

When an optical fiber exhibits only pure circular double refraction andlight in any polarization state is injected into said fiber, forexample, with elliptical polarization by a point 80 of Poincare sphere61, the polarization states occurring along the optical fiber arelocated on a circle 81 parallel to equator 62 and progress along saidcircle at an angular velocity determined by the value of vector 78.

If optical fiber 41 exhibits intrinsic linear double refractionrepresented for example by vector 77, and if, additionally, circulardouble refraction is impressed thereon by suitable torsion representedby vector 78, then generally fiber 41 exhibits elliptical doublerefraction resulting in FIG. 5 from the addition of vectors 77 and 78 toa resultant vector 82 which corresponds on the sphere surface to aninduced polarization state 83.

If, instead of the left circular double refraction represented by vector78, an equal right circular double refraction is impressed by vector 79on fiber 41, in combination with the linear inherent double refractionrepresented by vector 77, the result is an elliptical double refractionrepresented by a vector 84 and the induced polarization state at a point86 of sphere 61, said point being symmetrical to induced polarizationstate 83 relative to equatorial plane 66.

If light of, for example, linear polarization state 74 is injected intofiber 41, the polarization states occurring along said fiber 41 arelocated in one case on a circle 87 concentric to the polarization state83, and, in the other case, on a circle 88 concentric to polarizationstate 86. In the selected special case said circles 87 and 88 arecovered counterclockwise at the same angular velocity indicated by thevalue of vectors 82 and 84.

The length of the portions 58 and 59 of coupling sections 51 subjectedto torsion-induced elliptical double refraction η is determined as onehalf of λ length l.sub.ηk applicable to the elliptical double refractionassumed to be given and which is shorter than the λ length l.sub.λ indelay sections 48, 49, and 50 of fiber 41, where only linear doublerefraction β is present. Considering equations 12 and 13, with a givendegree of torsion τ and a given linear double refraction β the ratiol.sub.λk /l.sub.ηk of these λ lengths is given by the equation: ##EQU9##when g=0.13. Then the linear polarization state along portions 58 and 59varies along a π arc of circles 87 and 88 extending around inherentpolarization states 83 and 84.

Further to clarify the operation of filter 40 of FIG. 4, reference ismade to FIG. 6, which is a view of Poincare sphere 61 seen in thedirection of arrow 89. Without limiting the general concept it isassumed that the intrinsic double refraction of first delay section 48is the horizontal state 72, and that the azimuth 2φ_(B) of apolarization 74 injected into the first delay section is double theellipticity +2Ψ_(B) of the two elliptical inherent polarization states83 and 86 of about 4°. Then the polarization state develops as followsalong optical fiber 41. Light of wavelength λ_(k), injected withpolarization state 74 into optical fiber 41, after covering a distancep·l.sub.λk in first delay section 48, exhibits again the samepolarization state 74 determined by input polarizer 54, after coveringcircle 76 p times along this distance. In the rest of length l.sub.λk /4to first clamping point 42 followed by first coupling section 51extending to an end section 91, the polarization changes from linearpolarization state 74 to polarization state 92 located on the greatcircle passing through poles 63 and 64. In the following first portion58 of first coupling section 51, in which fiber 41 imparts leftelliptical double refraction with inherent polarization state 83, thelight polarization changes from elliptical polarization state 92 along asemicircle 93 with angular radius 2Ψ_(B) to polarization state 72 whichis a horizontal polarization in the special example. In the followingsection portion 59 of first coupling section 51 the polarization changesfrom linear polarization state 72 along second semicircle 95 with anangular radius 2Ψ_(B) on unit sphere 61 to a right ellipticalpolarization state 94 located under equatorial plane 66 and extending tothe second clamping point 42 of first coupling section 51. In theinitial section 96 of length 3/4l.sub.λk following said clamping point42 and second delay section 49 the polarization of the light passingthrough optical fiber 41 changes again from right ellipticalpolarization state 94 along circle 76 which extends concentricallyaround polarization state 72 to a linear polarization state 97 locatedin the left hemisphere of the Poincare sphere 61 in FIG. 6, and fromwhich the polarization state 72 introduced into optical fiber 41 has anazimuth 4Ψ_(B) =2φ_(B). After the additional length p·l.sub.λk of seconddelay section 49, the same polarization state 97 is obtained again afterp cycles along circle 76. In the following end section 98 of lengthl.sub.λk /4, extending to the next clamping point 42, the polarizationchanges from state 97 to right elliptical polarization state 94 alongcircle 76. In the first portion 58 of the next coupling section 51optical fiber 41 exhibits again the same left elliptical doublerefraction as in the first portion of the first coupling section, andthe polarization changes from right elliptical polarization state 94,and a left elliptical polarization state 99 develops in the middle ofthe second coupling section 51 along a semicircle 101 extending on thesurface of Poincare sphere 61 concentrically to left elliptical inherentpolarization state 83 and having an angular radius 6Ψ_(B) or 3φ_(B).

In the following second portion 59 of second coupling section 51 thelight polarization changes from left elliptical polarization state 99 toa right elliptical state 102 along a semicircle 103 concentric to thedirectional axis of the vector of right elliptical inherent polarizationstate, and has an angular radius of 10Ψ_(B). From this right ellipticalstate 102 the polarization changes in the following, third delay sectionwhose length is equal to that of the second in the initial section 96 oflength 3/4l.sub.λk along a 3/4 circle 104 concentric to horizontalinherent polarization state 72, to a linear polarization state 106 whichhas again moved the further azimuthal distance 8Ψ_(B) or 4φ_(B) as thepolarization state 97 reached at the corresponding point of thepreceding delay section.

In the next portion of length pl.sub.λk of the third delay section 50the polarization changes along a complete circle 104 coveredcounterclockwise p times. The variation in the polarization state alongthe next delay section 49 occurs exactly as explained above, and onlythe radius of the circles concentric to elliptic polarization states 83and 86 and linear polarization state 72 varies in the manner illustratedin FIG. 6, to which reference is made for the details. This last delaysection 50 is shorter by l.sub.λk /4 than the preceding delay section49, so that the light decoupled at the end of the fiber by a collimatorlens 107, an output polarizer 108 similar to polarizer 23 in FIG. 1, anda suitable photoelectric detector 109, exhibits a linear polarizationstate.

The azimuthal distances between linear polarization states 74, 97, 106,etc., in effect at the end of each delay section are 8Ψ_(B) or 4φ_(B).Therefore, the effect of coupling section 51 of elliptical doublerefraction induced by twisting between delay sections 48, 49, and 50, istotally identical to that of the fanlike orientation of the optical axes26 of individual crystal plates 21 in the Solc filter of FIG. 1 when theazimuthal distance between the linear polarization states in effect atthe outputs of the individual delay sections is adjusted by appropriatesetting of the degree of torsion τ according to equations (5) and (6).As directly apparent from FIG. 5, 2Ψ_(B) is determined by the equation:##EQU10## or, in consideration of equation (13) ##EQU11## for g=0.13.The value 2π/m of said azimuthal distance to be set according toequation (6) must be equal to 4 times the value of the angle indicatedby equation (16), so that: ##EQU12## The length of coupling section 51is then established according to equation (14) by the degree of torsionas in equation (17).

A linear double refraction with the orientation required for a filtereffect of the type of a Solc fan or folded filter according to equation(5) or (7) can also be impressed on an optical fiber 110 by pinchingand/or bending. When the fiber is pinched between jaws 111 and 112 asshown in FIG. 7 with a force applied at a right angle to itslongitudinal axis 113 as symbolized by opposite-direction arrows 114 and116 in FIG. 7, optical axes 117 and 118, along which the fiber exhibitsthe greatest or the least index of refraction, extend in the direction114, 116 of the applied pinching force and at a right angle thereto. Inthe case of a fiber 110 of 100 μm diameter the pinching force necessaryto obtain a double refraction of about 2π/m is about 1 N. This force maybe localised or distributed along a greater length of the oppositesurface lines of the fiber.

When, for example, an optical fiber 120 is curved, especially in theform of a loop as shown in FIG. 8, and laid in one plane 121, two axes122 and 123, corresponding to the extreme values of the effective indexof refraction, are present. Axis 122, associated with the maximum valueof the index of refraction, extends in the radial direction of theeffective radius of curvature, and axis 123, associated with the minimumvalue of the index of refraction, is perpendicular to the plane ofcurvature 121. The value of the linear double refraction resulting fromthe flexure of fiber 120 depends on the fiber radius of curvature, andcan therefore be adjusted by a suitable choice of the radius.

In solid-core fibers, the linear double refraction is approximatelyproportional to the square of the reciprocal radius of curvature. Inaddition, it also depends on the diameter of the fiber itself. A doublerefraction of about 4π/loop is obtained in a quartz-glass fiber laid inclosed circular loops of about 10 cm diameter when the fiber diameter isapproximately 100 μm. FIGS. 9 and 10 show the basic structure of apinching device 124 and the successive, equal-length sections of anoptical fiber 110 exhibiting equal amounts of linear double refraction.The device is capable of imparting the orientation of double-refractionaxes 117 and 118 necessary to obtain a fiber-optics Solc folded filteraccording to equation (6).

Pinching device 124 comprises two pinchers 126 and 127 shown in planview in FIG. 9 and in a view in the direction of the longitudinal axis128 of optical fiber 110 in FIG. 10. The details of said device requireno specific explanation. Pinchers 126 and 127 can be placed fromopposite sides on straight optical fiber 110, so that the pair ofpinching jaws 129, 130 of pincher 126 and the pair of pinching jaws 131,132 of the other pincher 127 alternately engage optical fiber 110, asviewed in the longitudinal direction of said fiber. The median planes133 and 134 of the pincher gap, evidenced by the central axis 128 of theoptical fiber and the pivot axes 136 and 137 of the jaws 129-132 of thetwo pinchers 126 and 127, enclose an angle of 180°-ξ, where ξ is appliedby equation (6), i.e. amounts to 15° in the illustrated embodimentcomprising a total of 6 alternating pairs of pinching jaws 129, 130 and131, 132. The pinching forces applied in the direction of arrows 138,139 and 140, 141 are defined for example by tension screws 142 and 143engaging jaws 126 and 127. It is apparent that the pinching device andthe adjustment and orientation of the pinching forces applied to opticalfiber 110 can also be provided by other equivalent means.

A similar arrangement for the production of a fiber-optics Solc foldedfilter is represented in FIG. 11. The double refraction necessary to thefilter effect is impressed here on an optical fiber 120 by bending thefiber. Optical fiber 120 is arranged in closed loops 144 and 145 whichoverlap alternately in different loop planes 146 and 147 enclosing anangle of 180°-ξ. Each loop 144 or 145 corresponds to a delay section ofdetermined length. Instead of one loop, groups of loops comprising aplurality of loops can be provided to form a delay section.Advantageously, to position optical fiber 120, loops 144 can be placedin one loop plane 146, and loops 145 in the other loop plane 147, inopposite looping directions.

It is understood that the above-cited means to obtain the necessarydouble refraction and/or to connect the double-refraction delay sectionscan be used in combination. For example, a Solc fan filter can beobtained by applying the pinching forces in fanlike distributeddirections of action to an extended optical fiber. It is possible alsoto connect delay sections to which the necessary double refraction isimparted by bending--the loops or bends of the individual delay sectionsbeing in a common plane--coupling sections obtained as described inreference to FIG. 4. A fiber-optics Solc fan filter with looped delaysections may also be produced by arranging the loops in fan- orstar-shaped planes.

FIG. 12 represents schematically a fiber-optics arrangement 150consisting of optical-fiber transmission filters of the invention, whichin particular may present the structure described in reference to FIG. 4or FIGS. 9-11. Said arrangement provides for the decomposition intocomponents practically without loss of a signal light beam which isguided by a single optical fiber 151 and comprises a plurality ofpartial light beams of different wavelength, which are assumed to bemonochromatic within a narrow band width δλ. Arrangement 150 is usable,for example, in a fiber-optics communications transmission system toseparate and process individually partial light beams of differentwavelength used as carriers for different information and normallypulsed in a manner characteristic of the information concerned.

In the illustrated embodiment of FIG. 12, it is assumed for simplicitythat the signal light beam 152 comprises four partial light beams whosespectral intensity distribution around central wavelengths λ₁, λ₂, λ₃,and λ₄ are represented by narrow-band intensity maxima 153 in the firstfamily of curves of FIG. 13. It is assumed also that the partial lightbeams guided by an optical fiber 151 are fully polarized. Arrangement150 contains four fiber-optics transmission filters 154-157 whosetransmission behavior is adapted to the spectral distribution of theparallel light beams as apparent from FIG. 3, so that only onetransmission maximum 32 (see FIG. 3) of filters 154-157 coincides withone of the signal light waves λ₁ -λ₄. The illustrated variation of thetransmission curves related to filters 154-157, preferably in the sameorder, can be obtained by suitable selection and/or tuning of thedouble-refraction properties of the optical fibers.

The structure of a filter arrangement 150 as illustrated in FIG. 12, isset up so that signal light beam 152 containing all the partial lightbeams impinges first on filter 154 whose effective transmission maximum36 is adjusted to wavelength λ₁. The linear polarization state of thelight injected into first filter 154 necessary for the appropriateoperation of said filter is obtained by the suitable adjustment of anoptical phase compensator, for example a Soleil-Babinet compensator 158.The plane of polarization of the light issuing from compensator 158 andinjected into the optical fiber of first filter 154, forms the referenceplane for the directions in which the above-cited double-refractionproperties are imparted to the optical fibe of filter 154 by lateralcompression, bending, or the like. The output polarizer of first filter154, comparable to the output polarizer 108 of FIG. 4, forms apolarizing prism which, for example in the extraordinary output path,deflects the partial light beam of wavelength λ₁, to which first filter154 is tuned, to a first detector 160, and, in the ordinary output path,injects the residual partial light beams of wavelengths λ.sub. 2, λ₃,and λ₄ with a linear polarization state, into the optical fiber ofsecond filter 155. The output polarizer of second filter 155 or theinput polarizer of third filter 156 forms another two-output polarizingprism 161 which directs the partial light beam of wavelength λ₂ toanother detector 162 and the residual partial light beams of wavelengthsλ₃ and λ₄ to third filter 156. A third two-output polarizing prism 163constituting the output polarizer of third filter 156, finally, over itsextraordinary and ordinary output light paths, provides for thenecessary spatial separation of the residual partial light beams ofwavelengths λ₃ and λ₄ for the separate informations with detectors 164and 166. A fourth filter 157, shown by broken lines in FIG. 12, isneeded only when filters tuned to other wavelengths (λ₅, . . . λ_(n))must be connected in a similar way over an additional polarizing prism167. Polarizing prisms appropriate to arrangement 150 are, for example,the Rochon prism, the Senarmont prism, and the Wollaston prism.

A system adapted to the same application as the arrangement 150 of FIG.12 can be obtained by introducing after optical fiber 151 guiding thetotal light beam a multiple beam splitter device to divide primary lightbeam 152 into a number of spatially separate light beams ofapproximately equal intensity, corresponding to the number of partiallight beams of different wavelength. In this case a narrow-bandtransmission filter of the invention, adapted to the wavelength to befiltered, is provided in the path of each of said light beams.

It is to be understood that the above-described arrangements are simplyillustrative of the application of the principles of this invention.Numerous other arrangements may be readily devised by those skilled inthe art which embody the principles of the invention and fall within itsspirit and scope.

I claim:
 1. An optical transmission filter comprising at least onedouble-refraction element mounted between polarizers which determinecharacteristic polarization states, wherein the dimensions of thedouble-refraction element, viewed in the direction of a predeterminedlight path, are sufficient to obtain a phase delay of at least 2πbetween orthogonal polarization states for at least one wavelength oflight injected along the direction of the light path, characterized inthat the double-refraction element is a single-mode optical fiber whosedouble-refraction characteristics are sufficiently weak so that a λlength within which the light beams propagating with orthogonalpolarization states in the fiber are mutually delayed by 2π, is at least1 cm.
 2. The transmission filter as in claim 1, characterized in thatthe single-mode fiber exhibits a linear double refraction, with thedifference Δn between indices of refraction of the light propagating inthe fiber for the two orthogonal polarization states being at least10⁻⁸.
 3. A fiber-optics arrangement for communications transmission,comprising:an optical fiber for guiding a signal light beam which, ascarrier for different information signals, covers different wavelengths;and a number of optical filters separating and decoupling individualpartial light beams; characterized in that the outputs of the opticalfilters are coupled to beam splitting means for dividing a primary lightbeam containing the partial light beams into a number of spatiallyseparate light beams of equal intensity corresponding to the number ofpartial light beams, and in that a narrow-band transmission filteradapted to the different wavelengths to be filtered is arranged in eachof said spatially separate light beams, wherein each of said narrow-bandtransmission filters comprises at least one double-refraction elementmounted between polarizers which determine characteristic polarizationstates, wherein the dimensions of the double-refraction element, viewedin the direction of a predetermined light path, are sufficient to obtaina phase delay of at least 2π between orthogonal polarization states forat least one wavelength of light injected along the direction of thelight path, characterized in that the double-refraction element is asingle-mode optical fiber whose double-refraction characteristics aresufficiently weak so that a λ length within which the light beamspropagating with orthogonal polarization states in the fiber aremutually delayed by 2π, is at least 1 cm.
 4. A fiber-optics arrangementfor selecting monochromatic light beam components of differentwavelength λ₁, λ₂, λ₃, . . . λ_(n) from a light beam containing aplurality of such components, using a fiber-optics transmission filtercomprising at least one double-refraction element mounted betweenpolarizers which determine characteristic polarization states, whereinthe dimensions of the double-refraction element, viewed in the directionof a predetermined light path, are sufficient to obtain a phase delay ofat least 2π between orthogonal polarization states for at least onewavelength of light injected along the direction of the light path,characterized in that the double-refraction element is a single-modeoptical fiber whose double-refraction characteristics are sufficientlyweak so that a λ length within which the light beams propagating withorthogonal polarization states in the fiber are mutually delayed by 2π,is at least 1 cm, and further characterized in that at least one saidfilter is provided, into which a light beam containing at least some ofthe monochromatic light beam components is directed, wherein the outputpolarizer of said filter comprises a two-output polarizing prism throughwhich an output light path of a partial light beam of the wavelength towhich the filter is tuned, is directed to a detector which injects areduced partial beam over a second output light path of the polarizingprism also constituting the input polarizer of a second filter into thefiber, and wherein the output polarizer of said second filter is asecond two-output polarizing prism, the output light beam of the secondpolarizing prism containing only light to which the second filter isadjusted, being directed to a detector, and the other output light beambeing also directed to a detector in a case where is still containsexclusively monochromatic light, and a third filter connected to thefiber to separate a partial light beam of specific wavelength in a casewhere the output light beam does not contain exclusively monochromaticlight.
 5. The fiber-optics arrangement as in claim 3 or 4, characterizedin that the fiber-optics transmission filters utilized to filter theindividual partial light beams and in which the wavelengths to befiltered are used in the same order, the wavelength interval between thepartial light beams being larger than the bandwidth of the filtertransmission maxima, and the spectral range over which the partial lightbeams are distributed, is narrower, by at least the bandwidth of thetransmission maxima of the filters than the shortest wavelength intervalΔλ_(k) between two transmission maxima.
 6. The transmission filter as inclaim 1, 2, 3 or 4, characterized in that a linear double refraction isimpressed on the optical fiber by pinching at least one portion of saidfiber.
 7. The transmission filter as in claim 6, characterized in thatthe optical fiber is placed between polarizers marking the samepolarization state and pinched at a number of points m along a portionof its length, the directions in which the pinching forces are appliedbeing distributed in a fanlike manner about the fiber axis, and theangular distances ω_(j) of these directions of application from theplane of polarization of the polarization states impressed on the fiberbeing given by the equation:

    ω.sub.j =(ξ/2)+(j-1)ξ

where j=1, 2, . . . m (an integer) and ξ=(π)/(2m).
 8. The transmissionfilter as in claim 6, characterized in that the optical fiber isarranged between crossed polarizers and pinched at a number m of pointsdistributed along a portion of its length, the directions of applicationω_(j) of the pinching forces alternating relative to the plane ofpolarization of the impressed polarization states when viewed in thelongitudinal direction of the fiber, according to the equation:

    ω.sub.j =(-1).sup.j+ 1(ξ/2)

where ξ=π/2m and j=1, 2, . . . m (integers).
 9. The transmission filteras in claim 1, 2, 3, or 4, characterized in that said optical fiber isbent to impress double refraction.
 10. The transmission filter as inclaim 9, characterized in that the optical fiber is bent into loops. 11.The transmission filter as in claim 9, characterized in that sections ofan optical fiber material containing no inherent double refractionarranged between parallel polarizers are formed into loops overlappingin different planes intersecting along a central axis and whose angularposition relative to the direction of polarization of the incident lightis given by the equation:

    ω.sub.j =(ξ/2)+(j-1)ε

where ε=π/2m and j=1, 2, . . . m (integer), the fiber being placed inthe separate planes with the same number of loops and the samediameters.
 12. The transmission filter as in claim 9, comprising anoptical fiber extending between crossed polarizers and set up inabutting loops, characterized in that the loops or groups of equalnumbers of loops abutting in alternate looping directions, are placed indifferent planes which are symmetrical with respect to the planes ofpolarization of the polarization states impressed on the fiber, andenclose an angle π-ξ, where ξ=π/2m when m is the number of loops orgroups of loops present in one or the other plane.
 13. The transmissionfilter as in claim 12, wherein the loops are of equal diameter.
 14. Thetransmission filter as in claim 1, 2, 3 or 4, characterized in that theoptical fiber exhibits a circular or an elliptical double refractioninduced by torsion about its longitudinal axis.
 15. The transmissionfilter as in claim 14, characterized in that the optical fiber exhibitsa linear inherent double refraction β and is retained by supportsdistributed along its length, and further wherein the optical fiber isdivided into a number of delay sections and a number of couplingsections extending between pairs of delay sections, wherein the delaysections arranged between the coupling sections having the same lengthwhich is equal, for a determined wavelength λ_(k) of the incident light,to (p+1) times the λ length l.sub.λk to produce a 2π phase shift in theorthogonal polarization states, the length of a first delay sectionadjacent to a first coupling section being p+1/4l.sub.λk, and the lengthof a delay section between the last coupling section and an outputpolarizer being p+3/4l.sub.λk, wherein the coupling sections are twistedin their transverse median plane relative to fixed ends thereof with adegree of torsion τ given by the equation: ##EQU13## where ξ=π/2m and gis a constant factor ranging between 0.13 and 0.16 for quartz fibers,and in that the length l.sub.ηk of the coupling section is determinedaccording to the equation: ##EQU14##
 16. An optical transmission filtercomprising:an input polarizer for receiving light along a predeterminedlight path; an output polarizer; and a single-mode optical fiber mountedbetween said input and output polarizer, comprising at least one delaysection for producing a linear double refraction and at least onesection which is twisted under a torsional force to produce anelliptical double refraction.
 17. An optical transmission filtercomprisng:an input polarizer for receiving light along a predeterminedlight path; an output polarizer; and a single-mode optical fiber mountedbetween the input and output polarizers comprising: an input lineardouble refraction delay section for receiving light from the inputpolarizer; an output linear double refraction delay section for sendinglight to the output polarizer; an intermediate linear double refractiondelay section located between the input and output delay sections; and aplurality of coupling sections for coupling the input and output delaysections to the intermediate delay section, wherein the couplingsections are twisted under a torsional force to produce an ellipticaldouble refraction.
 18. A filter as in claim 16 or 17, wherein eachtwisted section of the optical fiber includes a first portion twisted toproduce a right elliptical double refraction and a second portiontwisted to produce a left elliptical double refraction.